I thought I would share a few bits of information with folks who aren't familiar with all of these games. I'm not a pro, and I can't comment on 1-2 yet, but hopefully this will help. I'll spare the generic tips about taking notes and such, though, and only mention what's relevant specifically to these games that might not be immediately obvious. There are no spoilers here, just a few pointers on mechanics.
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Save as much gold as you can. Training costs will start to escalate rapidly near the end of the game as you stand atop a mountain of experience points. You don't need to be cheap - by all means, buy yourself the spells and equipment you need - just be conscious.
In 3-5, banks will pay weekly interest on both gold and gems, so deposit as much as you can. In 6, get someone to master the merchant skill so s/he can buy and sell for the party at better prices, but I recommend at least buying the skill for everyone, because it will reduce your training costs and pay for itself very quickly. Later on you might want to expert it, once you can spare 9 skill points for it. I don't recall if merchant reduces training costs in 3-5, but it might be worth getting for everyone just for the sake of convenience. You can also make some extra cash with the Enchant Item spell, if you're so inclined.
In all likelihood, you won't be able to max out your training before you finish the game, but that's okay. You don't need those extra levels to win - they are there to give completionists something to do :p
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In Might & Magic III, you will probably want a druid or ranger, as only they can cast Water Walk. If you don't have one, you'll need to find items to cast the spell, bring along a hireling who can cast it, or make do with Teleport once you have it (I don't remember if water walking is strictly required to finish the game or even see all the areas, but it's very useful). The default party is pretty well balanced except for this one point, so if you want to go with it, I recommend replacing the barbarian with a ranger - they're good melee fighters.
In World of Xeen, the default party is pretty much perfect. Seriously, they've got all the bases covered and will do very well.
If you want to create your own party, don't spend too much time trying to get perfect stats, like you might in a D&D or Wizardry game. Your stats will rise to very high levels over the course of the game, and a few points here and there won't count for much.
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You might not notice at first, but training takes time. In 3-5, visiting any vendor or building takes one day (seriously), and training a character takes one additional day per character, but the number of levels you train at once doesn't matter. In 6, training takes one week per level, but everyone can train at the same time, so if you train one character 2 levels and everyone else 1 level, you'll be there for two weeks.
Honestly this isn't too important in 3-5, although your characters do age, but in 6 there are some things you'll want to watch the calendar for.
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On the subject of time, keep an eye on your characters' age. Especially in 3-5, some creatures can unnaturally age you, which can seriously damage your stats and potentially kill you. I once had a couple of characters suddenly die on me in the middle of travelling, and when I looked at their character sheets, they were over 120 years old! It's easy to miss if you're not paying attention. There are ways to reverse unnatural aging, but I'll leave it to you to discover them.

Since this seems a good thread for doubts and tips I will leave my question here:
How can I disarm traps in World of Xeen?

I'm not sure if you can. IIRC your Robber's thievery skill may affect the odds of avoiding it just like lockpicking chances. There's always clairvoyance so you can assess the risks.
Also the game autosaves when you visit the tavern (in the Xeen games at least), so be careful if you're planning on reloading your current savegame.
Finally, you'll find a pen and paper handy for this game from time to time. Not everything useful is logged in the notes thingy.

A few critical tips for Might and Magic 4-5 (or World of Xeen).
* Make sure you regularly sort out your items, as there is a major bug that can cause lost of item(s), due to lack of any treasure sorting screen. If all of your characters have as many items of a certain type (ie armor) as they can carry, then new items will actually delete old items.
* Store as much gold as you in the banks, in order to pay for highest levels of training. Training is really expensive for highest levels, when nearing the end of the game.
My characters were really old the last time I played through the game, as I wait (sleep) so long, in order to gain enough interest, to pay for later levels.

Just reread the manual, and actually Luck is the stat that comes into play regarding the avoidance of traps (at least in the Xeen games).

If you mean the traps on chests, just have your robber or ninja open it. Like Drat says, the chance of the trap going off depends on the thievery skill of the character opening it. If you don't have a robber or ninja, you'll have to eat the damage.
(EDIT: he's right again, Luck also comes into play)
You can't disarm world traps like pits or swinging blades, though. Use the Jump spell to get past them.
Hmm, was that in a particular version of the game? My experience was that if your inventory was full, the items would just disappear and you wouldn't pick them up at all.
Either way, though, it's an important thing to mention. Keep your robber's inventory clear.

Thanks for this, especially the tip about aging. Yikes!

Is there any way to control MM6 using just the keyboard? How aggravating having to move using the arrow keys and then talk to NPCs using the mouse. Are there hotkeys for dialogue options and other menus? I have mashed this whole keyboard with no luck.

If not, is there a way to rebind keys, or has anyone got a lead on a program that will remap your keyboard, either just while they are running or when certain games launch?

Edit: See this post. I haven't tried the patch myself but there's the answer to my question. (Is there a markup for strikethrough that works here?)

Spoiler links up ahead.

I find this Xeen quest guide handy from time to time.

Item spoilers - not locations or a how-to, just stats. How we were supposed to figure out this stuff on our own is your guess. I use this constantly.

A hints & tips page at the same website. This one says it's "low-spoiler" but there's actually a lot of information about how the world works which some might prefer to figure out on their own.

Not that I'm aware of, but I'm surprised you find it a problem. Once I found the hotkeys for attacks (A and S) and figured out that I could at least initiate dialogues, enter buildings and search fallen enemies with Space I didn't feel the need to use the mouse to be particularly impractical. But each to his own I guess...

What can I say, I'm a tragic hybrid of old-world keyboard fiend and MMO addict. If my hand is touching the mouse without moving the camera around, there's a horrible disconnect and I feel like I'm drowning.
Here is the Xeen manual in text form, because the PDF only pretends to be plaintext and searchable. Large chunks of it are not.

Some tips for Might and Magic I...
You start with a club and no armor, so you can die pretty easily. Your characters will miss lots of attacks, so it's a good idea to use the "sleep" spell. It works really well, and many times it will put all of your enemies to sleep. A sorcerer can use it. To cast it in battle, press C(ast), 1 (magic level) and 8 (sleep spell).
After defeating an enemy, use the "search" command, or else you'll never get any money.
Map EVERYTHING. According to the manual, all locations are 16x16, and when you use the a sorcerer's "location" spell it will tell you which direction you're facing and the position in the grid (like, x=8 y=6, for example). It's pretty easy to map places knowing this.

Unfortunately old items are deleted, and replaced with new items from the latest treasure hoard. So important items can easily be lost that way. If you lose the Sword of Xeen, you can ask a magic mirror 'I lost it' to get a replacement.
I have played the disk and CD versions of World of Xeen, and that bug occurs in all versions of the game.

I strongly recommend not playing without a barbarian. They're by far the best fighters in MM3-5, far stronger than knights (although an error in the manual tells otherwise).
If you play without one, you don't have the balance between might and magic. Barbarians get an extra attack every 4 levels, knights and ninjas every 5 , thieves and mixed classes every 6 levels. So at level 60 you have a 16-13-11 attack comparison and the gap is even bigger than it looks.
The monsters you meet at this point of the game will most likely be able to absorb the first 10 attacks with their AC while the remainig attacks do full damage. Strong monsters might absorb even the first 14 attacks, the barbarian will still do serious damage while the knight won't even be able to scratch them.
It's really a good idea to add a druid or ranger to the party, you can also find scrolls during the game which will crumble if you don't have a druid or ranger to learn a spell from them. But from the MM3 starting party I'd rather replace the knight or the paladin. I'd also consider replacing the gnome thief with a dwarf thief, you get +5 to thieving skill and +2 HP per level without disadvantage, but those are only little benefits.
One starting advice: There's a reason why there are walls in the city seperating your party from the monsters there, better gain some XP outside before starting to bash the walls.
In MM3 visiting the inn takes 1 day, but visiting stores only takes an hour, unlike in MM4+5.

Fair points. Personally, I didn't feel that I was short on melee power and I didn't seem to get good results from the barbarian class, but that's just me. It HAS been a while since I played MM3, as well.

NWC Dungeon in MM6 makes the start very much easier. You can get a +20 perm to all stats at the Shrine Of The Gods and then make Essence Of <insert stat here> at the unlimited potion counter in NWC. Effectively, this two gives you a +30 boost to all your stats.
If you prefer a challenge though, stay away!

The barbarian IS strong although I don't like it's limitations on weapons and armor. I usually only have 3 or 4 melee fighters and 1 needs to be a Paladin (for backup healing/raise dead if the Cleric croaks), 1 needs to be an archer (for secondary Lloyd's Beacon and late game extra Implosions) and 1 needs to be a Ninja/Robber (I prefer Ninja). Then either a ranger or a druid for Walk on Water and other Natural Magic, a cleric and a Sorcerer. There is no room for a barbarian in this setup.
He does pack a serious punch, but it's really not that hard to manage without him - Paladins, Archers, Ninjas, Rangers and Knights will all do more than adequate damage even if it'll be somewhat less.